Process for preparing and treating austenitic stainless steels

ABSTRACT

PROCESS FOR PREPARING AND TREATING NEW AUSTENITIC STAINLESS STEELS WITH IMPROVED CREEP STRENGTH AT HIGH TEMPERATURES, THE STEELS HAVING A COMPOSITION IN PERCENT BY WEIGHT WITHIN THE FOLLOWING LIMITS: CARBON $0.175 SILICON $1.0, CHROMIUM 15 TO 20, NICKEL 4 TO 16, MANGANESE 1 TO 12, MOLYBDENUM 0 TO 4, TUNGSTEN 1 TO 6, COPPER 0 TO 4, NITROGEN $0.15, VANADIUM AT LEAST EQUAL TO 1.2 TIMES NITROGEN WHERE VANADIUM PLUS NITROGEN $0.65, COLUMBIUM 0 TO 2, BORON 0.001 TO 0.005 AND THE BALANCE IRON AND INCIDENTAL IMPURITIES. THE PROCESS INCLUDES PREPARING THE ALLOY, WORK HARDENING THE ALLOY WHILE HOT, AND CHILING THE ALLOY. IN A PREFERRED EMBODIMENT COLUMBIUM IS MANDATORY IN AMOUNTS OF 0.2 TO 2.0 PERCENT AND PREFERABLY 0.4 TO 1.0 PERCENT.

March 14, 1972 DECROIX 3,649,376

PROCESS FOR PREPARING AND TREATING AUSTENITIC STAINLESS STEELS Filed Aug. 5, 1970 INVENTOR.

Y Jean Decroix B w M, ,M M L HIS ATTORNEYS United States Patent Ofice 3,549,376 Patented Mar. 14, 1972 Int. Cl. C21d 7/14 U.S. Cl. 148-12 8 Claims ABSTRACT OF THE DISCLOSURE Process for preparing and treating new austenitic stainless steels With improved creep strength at high temperatures, the steels having a composition in percent by weight within the following limits: carbon 50.175, silicon 1.0, chromium 15 to 20, nickel 4 to 16, manganese 1 to 12, molybdenum to 4, tungsten 1 to 6, copper 0 to 4, nitrogen 2015, vanadium at least equal to 1.2 times nitrogen where vanadium plus nitrogen 50.65, columbium 0 to 2, boron 0.001 to 0.005 and the balance iron and incidental impurities. The process includes preparing the alloy, work hardening the alloy while hot, and chilling the alloy. In a preferred embodiment columbium is mandatory in amounts of 0.2 to 2.0 percent and preferably 0.4 to 1.0 percent.

This is a continuation-in-part of application Ser. No. 741,245, filed July 1, 1968, and now abandoned and application Ser. No. 609,503, filed Jan. 16, 1967, now Pat. No. 3,551,142. The present invention relates to processes for preparing and treating new austenitic stainless steels having improved properties, especially improved creep strength characteristics at high temperature.

When a metal having a satisfactory creep strength up to approximately 650 C. is required, chrome-nickel steels improved by the addition of a variety of elements are frequently utilized. Some such additions, for example, titanium and aluminum, have the disadvantage that structural hardening must be made during heat treatment before the article is placed in service. Steels obtained in this manner are also costly and not readily weldable. Other additives, such as nitrogen and vanadium, cause a hardening precipitation in the course of creepage. Still other additives, such as boron, molybdenum, tungsten, copper, manganese and columbium, render the austenitic matrix more rigid at high temperatures.

All of these steels have creep characteristics that are improved when compared to the normal stainless austenitic steels, but in spite of this fact, they do not have sufficient creep strength when intended for working at temperatures exceeding 650 C.

The search for ever higher operating temperatures by certain industries, such as those for generating thermal power (superheater tubes), the manufacture of aircraft jet engines, and the manufacture of internal combustion engines (exhaust valves), has led to the frequent use of very special and very costly alloys which contain up to 20 percent chromium, nickel and cobalt (super alloys), or even 20 percent chromium, 55 percent nickel, 9 percent molybdenum, and 1 percent cobalt (Hastelloy X) for such purposes, or it has led to covering certain areas of the articles with alloys such as those referred to as stellites.

The present invention provides a process for preparing and treating new austenitic stainless steel which have creep strength characteristics above 650 C. which are comparable to those of the special alloys referred to above. These new steels are particularly useful for superheater tubes, components of aircraft jet engines, blades for gas turbines, steam turbines, and turboblowers, and valves for internal combustion engines.

Steels within the scope of this invention are stainless austenitic steels having a composition within the following limits stated as percent by weight:

Carbon $0.175. Silicon 51.0. Chromium 15 to 20. Nickel 4 to 16. Manganese 1 to 12. Molybdenum 0 to 4. Tungsten 1 to 6. Copper 0 to 4. Nitrogen 20.15 preferably 2.20. Vanadium 1.2 nitrogen. Nitrogen-l-vanadium 50.65. Columbium 0 to 2. Boron 0.001 to 0.005. Iron and incidental impurities Balance.

Within the limits specified above, the steels should contain at least one of the elements molybdenum and copper.

The preferred composition in percent by weight of steels Within the scope of this invention is as follows:

Carbon 50.130.

Silicon 51.0.

Chromium 15 to 20.

Nickel 7 to 16.

Manganese 1 to 5.

Molybdenum 1 to 3.

Tungsten 2 to 4.

Copper 0 to 3.

Nitrogen 20.15, preferably 2.20.

Vanadium z1.2 nitrogen. Nitrogen-i-vanadium 0.65. Columbium 0.2 to 2.0 and preferably 0.4 to 1.0. Boron 0.001 to 0.005. Iron and incidental impurities Balance.

The improved properties of the steels of this invention result from a combination of novel features in the austenitic steels including the composition limits and the process of treating these steels. These include the addition of either molybdenum or copper or both, in addition to tungsten in the specified limits, the addition of manganese in the specified limits, the particular nitrogen and vanadium contents and the relationship between them and the the addition of boron in the specified limits.

The relationship between the nitrogen and vanadium contents correspond to a balance between these two elements ofaddition, a balance that permits the precipitation of nitrides of vanadium N V which contributes to the hardness. The boron content is kept sufficiently low to avoid manufacturing difiiculties that a higher content would entail, whereby the steels of the invention can be produced without special precautions.

Further, I have now found that the addition of 0.2 to 2 percent and preferably 0.4 to 1 percent columbium to such steels substantially improves the mechanical properties of steels when they are work hardened while hot followed by controlled cooling to preserve the crystalline structure ob- TABLE I Stress (kg/mm?) Temperature C.):

Elongation at creep fracture is at least equal to percent at the end of 5,000 hours, and the elastic limit E is at least equal to 32 kg./mm.

The figure shows a plurality of stress curves in which the stress causing creep fracture in 10,000 hours is plotted as a function of temperatures.

In the drawings curves 1 to 5, which are drawn in full lines, represent known alloys. All of the alloys represented are intended for use in the manufacture of superheater tubes. Curve 1 represents a steel having a composition of percent chromium, 15 percent nickel, molybdenum, tungsten, nitrogen and columbium. Curve 2 represents a steel having a composition of 16 percent chromium, 10 percent nickel, 6 percent manganese, molybdenum, vanadium and columbium. Curve 3 represents a steel having a composition of 17 percent chromium, 14 percent nickel, molybdenum, copper, columbium and titanium. Curve 4 represents a super alloy having a composition of percent chromium, 20 percent nickel, 20 percent cobalt, molybdenum, tungsten, nitrogen and columbium. And curve 5 represents a Hastelloy having a composition of 20 percent chromium, percent nickel, 9 percent molybdenum and 1 percent cobalt.

Curve 6, drawn in broken lines, represents a steel of this invention intended for use in the manufacture of superheater tubes which will not be subject to corrosion when hot. The steel had a composition within the limits set forth below in Example 1.

It is clear from the drawing that while the steel represented by curves 1, 2, 3 and 6 and the super alloy represented by curve 4 are nearly identical with respect of creep strength at 650 C., the super alloy represented by curve 4 and the steel of the present invention represented by curve 6 are distinctly superior as soon as the temperature of utilization rises to 700 or 750 C. While the super alloy represented by curve 4 is slightly superior to the steel of this invention at these temperatures, its cost is also much greater. The Hastelloy represented by curve 5, while still superior at 700 C., is surpassed at 750 C. by the steel of the invention of curve 6, and the steel of this invention is also much less expensive.

The end use for which the steels of this invention are intended may dictate a specific composition within the scope of the general composition set forth above. Certain qualities in addition to improved creep strength may be useful, and therefore, in each instance the composition of steel will be selected that has the desired qualities in addition to the improved creep strength. Such qualities, for instance, may be stress rupture elongation, resistance to corrosion caused by the ashes of impure fuels or by organic lead salts, and elastic limit.

The following specific examples will illustrate the selection of specific steels coming within the scope of this invention for specific uses.

EXAMPLE 1 Steels suitable for superheater tubes operating above 650 C. require, in addition to good creep strength, stress rupture elongation and, on occasion, resistance to corrosion when hot. The elastic limit at ambient temperature is of secondary importance. The preferred composition of such steels are set forth in Table II in percent by weight.

TABLE II In the presence of corrosion when hot Manganesm 1 to 3.

Molybdenum" 0 to 2 1 to 3.

Tungsten 2 to 4 2 to 4.

Copper 1 to 4 0 to 3.

Nitrogen 015 (with V-i-N 20.15 (with V-l-Nz 0to1 01:01 0.001 to 0.005 0.001 to 0.005. Balance Balance. impurities.

EXAMPLE 2.

Steels used in the manufacture of components of aircraft jet engines require a high elastic limit. For such uses the preferred composition in percent by weight of steels within the scope of this invention is as follows:

Carbon 50.130.

Silicon g1.

Chromium 15 to 20.

Nickel 7 to 16.

Manganese 1 to 5.

Molybdenum 1 to 3.

Tungsten 2 to 4.

Copper 0 to 3.

Nitrogen 20.15, preferably Vanadium 212x nitrogen with V+N s0.65.

Boron 0.001 to 0.005.

Iron and incidental impurities Balance.

EXAMPLE 3 Steels well suited for the manufacture of exhaust valves for internal combustion engines require a high elastic limit and resistance to corrosion in the presence of the combustion products of organic lead salts. For such uses the preferred composition in percent by weight of steels within the scope of this invention is as follows:

Carbon 0.100 to 0.150.

Siliocn 50.30. Chromium 15 to 20. Nickel -s 4 to 8. Manganese 8 to 12. Molybdenum 0 to 2. Tungsten 1 to 3. Copper 0 to 3. Nitrogen 20.15, preferably 20.20 Vanadium 1.2Xnitrogen V+N s0.65. Columbium 0 to 2. Boron 0.001 to 0.005. Iron and incidental impurties Balance.

It is desirable that the composition contain both tungsten and copper, and possibly molybdenum.

In order to further increase the creep strength of steels of this invention at approximately 650 to 800 C., as well as to increase their elastic limit, they are treated by means of my special process which entails work hardening with hot, followed by chilling which brings the metal back to ambient temperature while preserving the crystalline structure obtained by the work hardening. In other words, the final condition for transformation that avoids total or partial self-recrystallization and self-restoration in the structure of the metal is selected. Further, these conditions are determined for each particular category.

Moreover, subsequent treatment and conditions of application should not alter the structure obtained by work hardening when hot.

Although it is generally a well known fact that work hardening increases the elastic limit when cold, as well as the creep strength of austenitic steel, it is noteworthy that applying such a treatment to the steels of this invention produces an improvement in their qualities at very high temperatures, an improvement that is distinctly greater than that obtained from the same treatment of other austenitic steels intended for the same uses.

The following example illustrates a steel within the scope of the present invention subjected to my special process described above and compares the improvement achieved by work hardening of steels within the scope of this invention with the improvement achieved by work hardening a prior art austenitic steel.

EXAMPLE 4 Two steels having the compositions, in percent by weight, set forth in Table III are intended for the manufacture of movable blades for gas turbines. Steel 7 is a steel within the scope of the present invention, while steel 8 is an austenitic steel having tungsten and titanium present.

TABLE III Steel 7 Steel 8 C arbon 0. 050 0. 100 Silicon 0.60 0.50 Chromium. 18 17. 5 Nickel 14. 5 13. 7 Manganese- 1. 75 1. 60 Molybdenum 2. 53 Tungsten 3. 42 3. Titanium 0. 625 Nitrogen- 0. 21 anadiu m 0.35 Boron 0. 003 0. 003 Iron and incidental impurities Balance Balance For both steels the work hardening when hot treatment consisted of a reduction in cross-section by 20 percent at 850900 C. Table IV gives the results of the measurement of the elastic limits E (in kg. per mm?) and the stresses (in kg. per mm?) which cause creep fracture at 750 C. at the end of 1,000 hours and 5,000 hours. It also shows the relative increase in these values in each steel due to the transition from the annealed to the work hardened conditions.

In cases where the steels of the invention are used in the form of sheets, it is desirable to manufacture them according to the process described hereinafter. This method of manufacturing makes it possible to improve still further the elastic limit E of the steels of this invention and their resistance to elongation by creeping in the course of the so-called period of secondary creepage. The sheets obtained will, therefore, be particularly suited to certain applications, such as in gas turbines and jet engines.

According to this process, a steel having a composition within the scope of this invention is rolled when hot. This is terminated by a pass of complete self-recrystallization into grains of controlled dimensions. This is then followed by cold rolling and, finally, a thermal annealing treatment at a temperature equal at the most to 1,050- C. and a stress relaxation treatment that does not cause complete self-recrystallization of the metal. In each case, depending on the analysis and application of the steel, it is necessary to determine the operational conditions of temperature, rate of reduction, and heating or cooling for each stage of the process. These conditions take into account the application, since it is preferable that the service at high temperatures that is required of the steel does not cause complete recrystallization.

The final hot rolling pass should eliminate the work hardening produced by the preceding passes and should produce in the steel the formation of grains in which dimensions are favorable for the continuation of the process. It is known that in order to obtain rather large grains, this latter pass must be carried out at a rather high temperature with a rather low rate of reduction. The rolling operation then causes a new work hardening of the metal. The latter stages are intended to relax the metal without causing complete recrystallization.

The following example illustrates a steel within the scope of the present invention which may be rolled into sheets and compares the properties of the steel sheet when obtained according to the process described above and when obtained by conventional methods.

EXAMPLE 5 A steel within the scope of this invention suitable for making steel sheets for gas turbines and jet engines has a composition in percent by weight as follows:

TABLE IV Steel 7 Steel 8 Relative Relative Work increase, Work increase, Annealed hardened percent, Annealed hardened percent E02, kgJmmfl:

An ambient temperature 38 82 +116 26 52 +100 At 650C 21 +138 16 34 +112 Stress of creep fracture at 750 C., kg./mm

After 1,000 hours 13.5 18 +33 10 13 +30 After 5,000 hours 10. 2 12. 5 +22 7. 3 8 +11 The comparison of the relative increases in the values Carbon 0.045 clearly shows that in steel 7 of this invention the work Silicon 0.55 hardening treatment brings about an improvement in its Chromium 18.8 properties of high temperatures that is much greater Nickel 15.2 than that which is expected with reference to the results Manganese 1.6 of the same treatment of a known austenitic steel 8. Molybdenum 2.47 The properties of the work hardened steel 7 of this Tungsten 3.61 invention are far superior to those of work hardened steel Nitrogen 0.20 8, which allows the former to be used at temperatures and Vanadium 0.37 with applied stress rates that are distinctly higher. Boron 0.003 It must be noted that steel 7 of this invention has an Iron and incidental impurities Balance elastic limit in tensile strength and a creep resistance at 700 C. equal to those of a titanium-aluminum steel with structural hardening, and at 750 C. it has a better creep quality than that of the latter steel in which hardening is no longer stable.

One sheet (sheet 9) of this composition was obtained by the conventional method comprising successively a hot rolling pass, an annealing treatment at 1100" C., a cold pass, and an annealing at 1100 C. A second sheet (sheet 10) was obtained by the method of this invention which comprises a hot rolling pass terminated by a pass at a temperature above 1000 C. with a reduction in thickness of from to percent, a cold pass with a reduction in thickness of from to percent, and an annealing treatment from a temperature on the order of 970-1000 C.

The following Table V shows the results of tests on sheets 9 and 10 giving the values for the elastic limit E (in kg. per mm?) measured at various temperatures, and the values of the elongation (in percent on an initial reference length equal to 5.65\/S, S being the section of the test piece) after deformation by creepage obtained by keeping it at 700 C. for 300 hours under a stress of 16 kg./mm.

It should be noted that the improvement in resistance to creep elongation was obtained without any appreciable change in the limit of creep fracture, and with a decrease in ductility at creep fracture of 40 to 20 percent on the average for a fracture caused at the end of approximately 800 hours. The presentation of the steel of the invention in sheets obtained in accordance with the process imparts to it an elastic limit and resistance to creep elongation which are on the same order of magnitude as those of a conventional structurally hardened austenitic steel containing titanium and aluminum.

While the processes discussed heretofore have resulted in a substantially improved austenitic stainless steel, I have found that still further improvements can be made by the careful control of the columbium content which is mandatory in this preferred embodiment. It is, of course, necessary to treat these preferred steels with the processes described hereinabove, namely work hardening while hot, followed by controlled cooling wherein the controlled cooling is done at a rate which brings the metal back to ambient temperature while at the same time preserving the crystalline structure obtained by the work hardening in the manner known to those skilled in the art.

The improved mechanical properties obtained by the addition of columbium and the heat treatment is clearly shown in the following test data. Two steels having the compositions set forth in Table VI were made. Steel 11 is made according to my invention and, therefore, contains columbium. Steel 12 is the same steel without the columbium.

Both steels were subjected to the same work hardening while hot and controlled cooling. A comparison of the following mechanical properties illustrates the advantages of my improved steels.

( A) Elastic limit E Table VII compares the elastic limits of steels 11 and 12 at 0.2 percent (in H bar) at ambient temperature and at 650 C. after a work hardening using the reduction ratios and temperatures specified.

It can be readily seen that the elastic limit E of steel 11 is always superior to that of steel 12. This improvement in elastic limit can be a much as 12 percent ambient temperature and exceeds 10 percent at 650 C.

(B) Resistance to rupture Table VIII compares the resistance to rupture due to creeping of steels 11 and 12. Creep tests were carried out at 700 C. and 780 C. and the results of stress from rupture due to creeping in 1000 hours. Steels 11 and 12 were tested which had been subjected to different work hardening treatments.

The tests indicated that the addition of columbium had a direct influence on the creepage at the highest temperature. The stress causing rupture at 780 C. after 1000 hours was improved about 15 percent.

(C) Ductility Table IX compares the ductility while hot of the two steels. The table gives the elongation of the test bars after rupture by creeping at 700 C. under 28 H bar for two different *work hardening conditions.

TABLE IX Average elongation Work hardening after ruptiurc (percent) conditions Steel 11 Steel 12 15% at 975 C 23 8 15% at 1,175 C 10 3 As shown, the ductility while hot is greatly improved by the addition of columbium.

While the present preferred embodiments of this invention have been described, it may be otherwise embodied within the scope of the appended claims.

I claim:

1. A process for preparing and treating a stainless austenitic steel having an improved creep strength above 650 C., ductility and corrosion resistance when hot comprising:

(A) preparing an alloy consisting essentially of up to 0.175 percent carbon, up to 1.0 percent silicon, 15 to 20 percent chromium, 4 to 16 percent nickel, 1 to 12 percent manganese, 0 to 4 percent molybdenum, 1 to 6 percent tungsten, 0 to 4 percent copper, at least 0.15 percent nitrogen, vanadium in an amount at least equal to 1.2 times the amount of nitrogen, the sum of vanadium and nitrogen not exceeding 0.65 percent, 0 to 2 percent columbium, 0.001 to 0.005 percent boron, and the balance iron and incidental impurities;

(B) work hardening said alloy while hot; and

(C) chilling said alloy to ambient temperatures while maintaining the crystalline structure obtained by work hardening to form said stainless austenitic steel.

2. A process as set forth in claim 1 wherein the work hardening comprises a reduction of approximately 20 percent at a temperature of about 850 C. to 900 C.

4. The process as set forth in claim 1 wherein the alloy includes up to 0.130 percent carbon, 7 to 16 percent nickel, 1 to percent manganese, 1 to 3 percent molybdenum, 2 to 4 percent tungsten, O to 3 percent copper and 0.2 to 2 percent columbium.

5. A process as set forth in claim 3 wherein the final pass is terminated at a temperature above 1000 C. with a reduction in thickness from 5 to percent, where said rolling pass gives a reduction in thickness from to percent, and the thermal annealing is conducted at a temperature of about 970 C. to 1000 C.

6. The process as set forth in claim 4 in which the columbium content is 0.4 to 1 percent.

7. The process as set forth in claim 4 wherein said work hardening is at a temperature of from 950 to 1200" C.

8. The process as set forth in claim 7 wherein said work hardening comprises a reduction from 10 to 20 percent.

References Cited UNITED STATES PATENTS 3,151,979 10/1964 Carney et al. 14812 3,216,868 11/1965 Nachtman 14812 3,284,250 11/1966 Yeo et a1. 14812 3,306,736 2/1967 Rundell 128 G 3,384,476 5/1968 Egnell 14812 L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD', Assistant Examiner U.S. Cl. X.R. 14812.4

Patent No. 3, 649, 376 Dated March 14, 1972 Inventor(s;) Jean DecroiX It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4 Line 57 l. 2- should read 1. 2-

Column 4 Line 57 After --ni trogen insert -with- Column 4 Line 69 -with-- should read -when- In Table IV Column 5 -An ambientshould read -A1: ambient--. Column 5 Line 63 -of high-- should read -at high- In Table VII Column 8 Line 6 l 075 C should read l, 075 C Claim 5 Column 9 Line 19 said rollingshould read said cold rolling- Signed and sealed this ll th day of November 1972.

(SEAL) Attest:

EDWARD Ma.lELEICHER,JRo ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents OHM P0 1050 (KO-69) USCOMM-DC GO3'76-Ffi9 r: u GUVENHMENI PRINTING OFFICE I96) 0-366-33d 

